Case definitions are sets of criteria used by public health agencies in the surveillance, or monitoring, of disease syndromes. In the United States, case definitions are established by the Centers for Disease Control and Prevention (CDC). One of the most controversial of all case definitions has been that for AIDS, which as a syndrome is characterized by more than two dozen different illnesses and symptoms as well as by specific indications on blood test findings.
The conditions included in the CDC AIDS case definition, most recently updated in 1993, are often called AIDS-defining illnesses. When diagnosed in a person with HIV, these conditions indicate that a person has progressed to AIDS. Under the 1993 case definition, an HIV-positive person also has AIDS when his or her CD4+ cell count has fallen below 200 cells per microliter or when CD4+ cells account for fewer than 14 percent of all lymphocytes (see Table, below). (In Canada, Western Europe, and most other locations outside the U.S., the same AIDS-defining illnesses are generally included in AIDS case definitions, but a formal AIDS diagnosis usually cannot be made on the basis of low CD4+ cell counts alone.)
In 1981, after reports of Pneumocystis carinii pneumonia, Kaposi's sarcoma, and other opportunistic infections in young gay men in San Francisco, New York, and Los Angeles, the CDC began surveillance for a newly recognized constellation of diseases eventually to be called AIDS. In 1982, the CDC developed a surveillance case definition for this syndrome focusing on the presence of opportunistic infections; it initially received case reports directly from both health care providers and state and local health departments. Once HIV was identified as the causative agent of AIDS and the epidemic became more widespread, state and local health departments assumed responsibility for AIDS surveillance. By 1985, all states and local governments had rules requiring health care providers to report AIDS directly to the state or local health department. These entities then report to the CDC, which in turn produces national surveillance data.
The AIDS case definition was expanded in 1985 to include a total of 20 conditions. Four of these conditions were cancers: Kaposi's sarcoma and three distinct types of lymphoma. The remaining conditions were opportunistic infections -- those caused by bacteria, fungi, protozoa, and other infectious agents -- that an intact immune system can usually manage but which take advantage of the "opportunity" provided by weakened immunity to proliferate in the body.
Ongoing evidence about the inadequacy of the case definition prompted another revision in 1987 and the inclusion of three additional conditions. One of the new conditions was an opportunistic infection, tuberculosis (TB), but only the extrapulmonary (outside the lungs) type. The other conditions were not opportunistic infections, but rather conditions resulting from the direct effects of infection by HIV of cells in the digestive system (wasting syndrome) and the central nervous system (encephalopathy or dementia). These three additions resulted in a one-quarter increase in reported AIDS cases, primarily among heterosexual African American and Latino individuals. These new cases also included high numbers of injecting drug users
By October of 1990, there were numerous studies linking HIV infection with a higher mortality and morbidity rate in persons with pulmonary TB and bacterial pneumonia, as well as a greater risk for an invasive form of cervical cancer. Further, the CDC definition was adopted as an eligibility criterion by U.S. government public assistance programs including the Social Security Administration. This was a problematic and inappropriate usage of the case definition, because the definition was intended for purposes of epidemiological surveillance and not to measure the disabling effects of HIV-related disease. The use of the CDC definition to measure disability thus became the subject of a 1990 class action lawsuit brought by individuals who were severely ill with HIV-related disease but who did not meet the CDC definition of AIDS. Physicians, activists, and, in particular, HIV-positive women began a movement to force the CDC to expand the surveillance definition of AIDS.
By the spring of 1992, the CDC began to consider expanding the surveillance definition to address a widely noted undercounting of certain populations, namely women, injecting drug users, and communities of color. The initial proposal by the CDC was to include all HIV-positive individuals with certain evidence of advanced CD4+ cell depletion. The CDC argued that this change would enable it to capture all those who were severely immunocompromised but were not suffering from one of the 23 opportunistic infections in the 1987 surveillance definition. Many HIV-positive individuals, activists, and health care providers, however, made the counterargument that abandoning a disease-based approach in favor of a surveillance system that was reliant upon the accuracy and availability of CD4+ cell count testing might lead to continued underdiagnosing and undercounting. In particular, advocates for women were concerned that this would perpetuate the medical community's historical failure to diagnose HIV-related illnesses in women.
Using the medical evidence collected for the Social Security litigation, a campaign was developed to lobby the CDC to add three additional conditions to the definition of AIDS. These were the three conditions for which there was the most medical evidence of recurrence and a more rapid advancement in the presence of HIV infection: invasive cervical cancer, recurrent bacterial pneumonia, and pulmonary TB. In November 1994, the CDC announced that it was expanding the surveillance definition, effective January 1, 1993, to include the three conditions from the community proposal and any HIV-positive individual with a CD4+ cell count of 200 or less or whose CD4+ cells represented less than 14 percent of all lymphocytes. Evidence for HIV seropositivity could be obtained by means of an HIV-antibody test, direct identification of the virus in tissues, an HIV-antigen test, or another highly specific licensed test for HIV. Two additional illnesses are also included in the case definition which are AIDS-defining in children but not in adults.
As a complement to, although not a replacement for, the AIDS case definition, systems have also been developed to classify the stage of HIV infection. The original CDC version used four groups marked I to IV to indicate, respectively, acute infection; asymptomatic infection; persistent generalized lymphadenopathy; and constitutional diseases, neurological diseases, secondary infectious diseases, secondary cancers, and certain other serious conditions. Another system developed in 1985 by the U.S. Army, the Walter Reed Staging Classification, used seven stages, ranging from stage 0, which represented a lack of infection and an intact immune system, to stage 6, which represented infection and advanced immune damage. (Early staging systems generally referred to HIV by the now-outdated name human T-cell lymphotropic virus-III/lymphadenopathy-associated virus [HTLV-III/LAV].)
The 1993 CDC system, in effect as of early 1998, makes use of three categories relating to the CD4+ cell count: Category 1 includes counts of 500 or more cells per microliter; Category 2 includes counts from 200 to 499; and Category 3 includes counts below 200 cells. The CDC staging system also contains three clinical categories for people who test HIV-positive. Category A includes individuals who have been asymptomatic except for persistent generalized lymphadenopathy and/or seroconversion syndrome. Category B comprises those who have never had an AIDS-defining illness but have had some of the less serious complications of HIV infection, including oral or vaginal candidiasis, constitutional symptoms such as fever or persistent diarrhea, oral hairy leukoplakia, herpes zoster, idiopathic thrombocytopenic purpura, listeriosis, peripheral neuropathy, cervical dysplasia, bacillary angiomatosis, or pelvic inflammatory disease. Category C is used to describe those who have had one or more of the AIDS-defining illnesses.
The use of both the CD4+ cell count and clinical categories provides a shorthand for where the patient stands in the course of the HIV/AIDS continuum. Thus, a person placed in Category A1 has the least immune damage and fewest clinical complications; someone scoring C3 is seriously ill. Anyone placed in Category 3 and/or Category C has an AIDS diagnosis under the 1993 CDC case definition. A somewhat different classification scheme is used for pediatric cases: three classes using the letter P for pediatric matched with the numeral 0, 1, or 2 to indicate the stage, along with subclasses and categories of specific types of diseases.
Since the 1993 revision of the AIDS case definition, there have been significant advances in tests such as the polymerase chain reaction (PCR) and branched DNA (bDNA) assay that determine the amount of HIV present in a person's blood (viral load). These and other ongoing changes in the state of knowledge about HIV/AIDS may prompt future revisions to the case definition.
The 1993 AIDS Surveillance Case Definition of the U.S. Centers for Disease Control and Prevention*
A diagnosis of AIDS is made whenever a person is HIV-positive and:
Additional Illnesses That Are AIDS-Defining in Children, But Not Adults
|* Entries on AIDS-defining illnesses can be found in the Encyclopedia of AIDS under the name given, unless otherwise noted in parentheses. Terminology may vary.|
† Added in the 1987 expansion.
‡ Added in the 1993 expansion.
AIDS, Pathogenesis of; Epidemics, Historical; Forecasting; HIV, Description of; Seroprevalence; Surveillance; Testing; United States Government Entries; Women
AIDS-defining illness, case definition, CD4+ cell count, surveillance; [specific AIDS-related illnesses by name]
Centers for Disease Control and Prevention, "1993 Revised Classification System for HIV Infection and Expanded Surveillance Definition for AIDS Among Adolescents and Adults," Morbidity and Mortality Weekly Report 41 (December 18, 1992), pp. 1-19.
Centers for Disease Control and Prevention, "Revision of the CDC Surveillance Case Definition for Acquired Immunodeficiency Syndrome," Morbidity and Mortality Weekly Report 36:supplement 1 (August 14, 1987), pp. 1S-15S.
Centers for Disease Control and Prevention, "Revision of the CDC Surveillance Case Definition of Acquired Immunodeficiency Syndrome for National Reporting -- United States," Morbidity and Mortality Weekly Report 34:25 (June 28, 1985), pp. 373-375.
Centers for Disease Control and Prevention, "Update on Acquired Immunodeficiency Syndrome (AIDS) -- United States," Morbidity and Mortality Weekly Report 31:37 (September 24, 1982), pp. 507-508, 513-514.
Huber, Jeffrey T., HIV/AIDS Community Information Services, New York, Haworth, 1996.
Osmond, Dennis H., "Classification and Staging of HIV Disease," in The AIDS Knowledge Base, 2nd ed., edited by P. T. Cohen, M. A. Sande, and P. A. Volberding, New York: Little, Brown, 1994.
The Encyclopedia of AIDS: A Social, Political, Cultural, and Scientific Record of the HIV Epidemic, Raymond A. Smith, Editor. Copyright © 1998, Raymond A. Smith. Carried by permission of Fitzroy Dearborn Publishers.
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